The present invention relates to a thickness control system for a hydraulically loaded rolling mill to ensure highly responsive thickness control for a workpiece.
FIG. 1 shows, as an example of coventional hydraulically loaded rolling mills, a single stand reverisble cold rolling mill 32 having coiling and uncoiling reels 20 and 27 on entry and exit sides. More specifically, a workpiece 30 to be rolled is fed from the reel 20 driven by a motor 19 and passes through a deflector roll 21 and is rolled between upper and lower work rolls 3 and 4. The workpiece 30 rolled passes through a further deflector roll 26 and is coiled by the reel 27 driven by a motor 28. The reel driving motors 19 and 28 are respectively associated with reel-motor tension controllers 18 and 29 so as to keep constant tensions of the workpiece on the entry and exit sides, respectively. Generally, the tension controllers 18 and 29 serve to control the tensions in proportion to motor currents. Rolling velocity or speed in a rolling line is controlled to a predetermined value by controlling work-roll driving motor 23 by a speed controller 24.
In FIG. 1, reference numeral 1 denotes a load cell for detecting a rolling pressure; 2 and 5, upper and lower back-up rolls; 6, a hydraulic cylinder for setting a roll gap between the work rolls 3 and 4; 8, a servo valve connected through a piping 7 to the cylinder 6; 9, a displacement gage for sensing displacement of a draft ram 6' in the cylinder 6; 10, a servo amplifier for transmitting a command in the form of a current signal to the servo valve 8; and 11, a coefficient multiplier for providing a control gain K.sub.G to amplify an output signal from a comparator 12 to control a draft position S' of the ram 6'.
In a basic position control loop, an instruction signal R is compared with an output signal S from the displacement gage 9 and a signal e representative of any deviation derived is multiplied by the gain K.sub.G in the coefficient multiplier 11. With the multiplied signal, the opening of the servo valve 8 is controlled through the servo amplifier 10 to quantitatively adjust a pressure oil supplied through the piping 7 to the cylinder 6, thereby controlling the position S' of the ram 6'. As a result, the lower back-up and work rolls 5 and 4 are displaced to adjust the roll gap between the work rolls 3 and 4 to a predetermined value. Thus, a hydraulic roll-gap control system 66 is provided.
Control of only the position S' of the ram 6' would cause error in the roll gap between the work rolls 3 and 4 due to elongation of the mill which have received the rolling pressure. To overcome this problem, compensation is made usually as follows. Reference rolling pressure P.sub.ref is stored at a proper timing after starting of the rolling. Difference .DELTA.P between the reference rolling pressure P.sub.ref and an actual rolling pressure during the rolling which is detected by the load cell 1 and is in the form of a signal P is calculated by a comparator or adder-subtractor 17 and then is divided by a mill modulus K.sub.m, which is specific in a mill just like a spring constant and has been detected in advance, in a coefficient multiplier 16 of a mill modulus control unit 54 to calculate an elongation of the mill. The calculated elongation is multiplied with a correcting gain c which will set a percentage of correction, thereby obtaining a modifying signal C.sub.p is given to modify the position S' of the ram 6'. The signal C.sub.p is given to the adder 13 as the instruction for the above basic position control loop to correct the position S' of the ram 6'. This procedure is generally called mill modulus control.
Further, in order to make absolute thickness of the workpiece 30 on the exit side of the mill into a desired or reference value h.sub.ref, a signal h representative of an actual thickness of the workpiece sensed by thickness gage 25 (or a thickness gage 22 in the case of a rolling in the reverse direction) on the exit side of the rolling mill 32 is compared with the reference value h.sub.ref by a comparator or adder-subtractor 31 to obtain a thickness deviation .DELTA.h. This deviation is passed through an integral controller 15 and is multiplied with a correction gain 1+(M/K.sub.e) for correction into an actual draft position in a coefficient multiplier 14 to obtain a modifying signal C.sub.h for correction of the position S' of the ram 6'. The modifying signal C.sub.h is also given to the adder 13 as an instruction of the above basic position control loop to correct the position S' of the ram 6'. This procedure is called monitor AGC. Here, M is a constant representative of hardness of the workpiece 30 and has been detected in advance. K.sub.e is a controlled mill modulus and will satisfy the equation: K.sub.e =K.sub.m /(1-c).
When the position S' of the draft ram 6' is changed to control the thickness of the workpiece 30 in the rolling mill in FIG. 1, the tensions applied to the workpiece 30 on the entry and exit sides fluctuate. For example, when the roll gap between the work roll 3 and 4 is narrowed down so as to decrease the thickness of the workpiece 30, the workpiece 30 will elongate and the tensions on the entry on the exit sides will decrease. Such fluctuation of the tensions may be absorbed by change of peripheral velocities of the reels 20 and 27 having much inertia; but, such absorptive response is generally slower by one or more digits than hydraulic roll-gap control. This means that, once the roll gap is changed and the tensions of the workpiece 30 on the entry and exit sides fluctuate, the tensions cannot be returned to preset values as quickly as the hydraulic roll-gap control. As a result, the decrease of the tensions on the entry and exit sides will cause deformation resitance on the workpiece 30 to apparently increase to nullify the narrowing of the roll gap, with a disadvantageous result that the workpiece thickness is not decreased. Namely, when attempt is made to decrease the workpiece thickness under the high-response hydraulic roll-gap control, the workpiece thickness cannot be thinned down at a rate higher than rate of responsive change of peripheral velocities of the reels 20 and 27. Therefore, disturbance of thickness on the entry side of, say, 2-3 Hz or more cannot be eliminated by hardening the mill by the above-mentioned mill modulus control since the thickness control is not responsive for the above reason mentioned.
It is often heard at rolling factories that thickness control accuracy cannot be improved as expected when the position S' of the ram 6' is controlled quickly by the hydraulic roll-gap control system 66. This will be attributed to the above reason.
FIG. 2 shows a computer simulation example done by the inventor, which will support the above-mentioned fact. An object simulated is the single stand reversible cold rolling mill shown in FIG. 1 where a workpiece with width of 1800 mm, entry side thickness of 0.52 mm, entry side setting tension of 1.36 tons and exit side setting tension of 2.35 tons is rolled at rolling speed of 1800 m/min. into thickness of 0.3 mm, roll gap being decreased midway and stepwise by 10 .mu.m. Assumption is such that response of hydraulic roll-gap control is 20 Hz with 90 degrees phase lag in frequency response and a desired value is reached with 0.04 second or less in step response. According to simulated results, thickness change .DELTA.h on the exit side reaches a steady value within about 1 second when roll gap is changed by 10 .mu.m. In the actual hydraulic roll-gap control system, a desired value in the system is reached with 0.04 second while the thickness is changed by 25 times as slow as this, which is attributed to the fact that the response in terms of change of peripheral velocities of the reels 20 and 27 on the entry and exit sides is slow as described above. That is, the reels 20 and 27, tensions of which are controlled by making the motor currents constant, have considerably great inertia including the motors 19 and 28 so that change of the peripheral velocities of the reels to some steady values for suppression of tension fluctuations is reached within about 1 second.
The present invention was made to overcome the above and other problems encountered in the prior art and provides a thickness control system for a rolling mill which can enhance the response of thickness control to attain product thickness at high accuracy.